Gas turbines produce a relatively clean hot exhaust gas stream. Modern gas turbine combined cycle (GTCC) power plants have a steam-Rankine bottoming cycle which is heated by that exhaust which produces about half as much power as the gas turbine. In order for the steam cycle to more closely match the temperature glide of the exhaust, and hence be more efficient, there are typically two or three boiling pressures, and one or more reheats, especially in the larger plants. Even with that level of complexity, there remain problems and inefficiencies. The boilings are at constant temperature, so that part of the heat acceptance necessarily departs from the temperature glide of the heat source.
In the low temperature range, the boiling pressure of steam is on the order of one to five atmospheres. Any lower boiling pressure would require much larger flow passages and bulkier, costlier equipment, to mitigate serious pressure drop penalties. Similarly, the heat rejection is at deep vacuum. Much design effort is required to make the turbine final stages, exit passages, and condenser inlet passages very large and with low-pressure drop. It has been pointed out that this condenser design condition (typically 101° F. and 7 kPa absolute pressure) prevents steam plants from taking much advantage of colder-than-design conditions. Vacuum operation also allows air in-leakage, making de-aeration necessary, which adds to the thermal losses. The deep vacuum condenser must be bulky, with large flow passages, and has low transfer coefficients and high pressure drop losses. Those conditions also mitigate against air-cooled condensers.
The four inter-related factors of constant pressure boiling, vacuum, pressure loss, and cost make steam plants not very effective in the low temperature regime. Any tabulation of the loss mechanisms of multi-pressure steam plants is dominated by the low-pressure components. The capacity and efficiency of conventional steam bottoming cycles are critically dependent on using lots of cooling water to maintain low vacuum, and the low vacuum in turn makes it extremely difficult and costly to accomplish dry cooling.
Ammonia Rankine cycles are well known in the prior art. They have been applied in ocean thermal energy conversion (OTEC) applications, and elsewhere using low temperature heat sources. A 22 MW experimental prototype of an ammonia bottoming cycle for a nuclear-powered steam cycle has been tested. Steam under vacuum (about 0.5 bar absolute) boiled the ammonia, which was expanded without superheating. Ammonia extraction vapor was designated for feed heating. The objective was to overcome the limitations of the conventional vacuum steam condensation. A more recent study analytically investigated a triple power cycle wherein gas turbine exhaust heated a steam bottoming cycle, condensing steam boiled the ammonia, and exhaust superheated the ammonia vapor. That disclosed cycle has several disadvantages, including use of extraction steam for feedwater heating; no steam reheat; and no preheating of feed ammonia or feedwater by the exhaust. Other prior art power cycles incorporate ammonia turbines for impure ammonia, e.g., U.S. Pat. Nos. 6,058,695, 6,194,997, 5,950,433, and 6,269,644. Given the high condensing pressure of ammonia, air-cooling is more readily achieved.
What is needed, and included among the objects of this invention, is a bottoming cycle for a gas turbine, i.e. a power cycle for input glide heat above about 600° F., which achieves higher efficiency by achieving a better glide match with the heat source, and which also avoids the disadvantages associated with vacuum operation.